1. Field of the Invention
The present invention relates to a step attenuator, and more particularly, to a step attenuator that can obtain a small attenuation value with input and output matching even when small-sized resistors and CMOS switches are used, and has broadband characteristics so as to obtain an attenuation value having multiple steps.
2. Description of the Related Art
In general, an attenuator is a kind of electromagnetic component that is used when a signal having high power is input, to attenuate the power of the input signal to an appropriate size. In particular, a step attenuator capable of determining an attenuation value in multiple steps is widely used in various kinds of receivers, such as a CATV receiver, which has a wide dynamic range. As compared with an analog control attenuator, a digital step attenuator obtains a more accurate attenuation value and exhibits better performance in an environment where temperature changes. Further, the step attenuator consumes less power and has relatively smaller distortion than the analog attenuator. In particular, in integrated circuit (IC) design, it is very important to accurately control a level of input power and transmit the controlled input power to a next block. To this end, the step attenuator that provides accurate control is used.
Generally, a basic circuit configuration of an attenuator is divided into a T-type configuration, a Pi-type configuration, and a bridged-T type configuration. In FIGS. 1A to 1C, the three configurations are sequentially shown. A step attenuator according to the related art includes the basic circuit configurations of the attenuators, shown in FIGS. 1A to 1C, which are connected in series with each other, and switches that bypasses the individual circuit configurations. MOS transistors are generally used as the switches.
When the bypass MOS transistor used in the step attenuator according to the related art is in an on-resistance (RON) state, it can be represented by an equivalent model with a value close to a zero resistance. However, as the MOS has the value closer to 0Ω, the size of the MOS transistor needs to increase, which causes an increase in manufacturing costs. The increase in size thereof causes an increase in parasitic capacitance, which narrows its frequency range. As a result, it is very difficult to practically realize a value of 0Ω by the MOS transistor. In the actual circuit, attenuation caused by the MOS transistor cannot be avoided. Despite variations in manufacturing process and size, the MOS transistor in the RON state has a resistance of several to several tens of ohms. Insertion loss according to the resistance of the MOS transistor in the RON state can be obtained by the following Equation 1.
                    IL        =                              -            20                    ⁢                                          ⁢                      log            ⁡                          [                                                2                  ⁢                                      Z                    0                                                                                        2                    ⁢                                          Z                      0                                                        +                                      R                    eq                                                              ]                                                          [                  Equation          ⁢                                          ⁢          1                ]            
In the Equation 1, Z0 is input and output impedance, and Req is an ON-resistance of the transistor. The unit of each of the two values is ohm. For example, when the input and output impedance is 50 ohms and the RON resistance is 10 ohms, the insertion loss is 0.827 dB.
Therefore, when the transistor is used to bypass a signal in the step attenuator, additional attenuation in the range of 1 to 3 dB occurs due to MOS transistors, parallel resistors, and various parasitic effects. For these reasons, it is very difficult to reduce the size of the step attenuator and realize a circuit that can obtain a small amount of attenuation of, for example, 5 dB.